1. Field of the Invention
The present invention relates generally to the conversion of carbon dioxide to oxygenates. In another aspect, the invention concerns a catalyst for converting a feed comprising carbon dioxide and hydrogen into methanol and dimethyl ether.
2. Discussion of Prior Art
Carbon dioxide for industrial use is typically recovered from synthesis gas production, substitute-natural gas production, cracking of hydrocarbons, and natural springs or wells. Although carbon dioxide can be used for numerous purposes such as, for example, refrigeration, carbonating beverages, and as an aerosol propellent, in certain situations it may be more desirable to convert carbon dioxide to other higher-value compounds such as oxygenates.
Oxygenates can be used for a variety of purposes such as, for example, enhancing of motor fuel octane and improving the emissions quality of motor fuel. Methanol and dimethyl ether are two oxygenates which can be of particularly high value. Methanol can be used for a variety of purposes including, for example, as an alternative motor fuel, as an intermediate in the production of high octane ethers (e.g., MTBE), and as a fuel for fuel cell driven vehicles. Dimethyl ether is also useful for a variety of purposes including, for example, as an alternative motor fuel, as a starting material towards the synthesis of various hydrocarbons, as a fuel additive to lower emissions, and as an aerosol propellent.
Although it is known that synthesis gas comprising carbon monoxide, carbon dioxide, and hydrogen can be converted into oxygenates using a copper-containing catalyst, there exists a need for a catalyst which is effective to convert carbon dioxide into oxygenates with increased carbon dioxide conversion as well as increased product selectivity towards methanol and dimethyl ether, particularly dimethyl ether.
An object of the present invention is to provide a catalyst and process for converting carbon dioxide to oxygenates wherein there is an increased carbon dioxide conversion.
A further object of the present invention is to provide a catalyst and process for converting carbon dioxide to oxygenates wherein there is an increased product selectivity towards methanol and dimethyl ether.
A still further object of the present invention is to provide a catalyst and process for converting carbon dioxide to oxygenates wherein there is an enhanced product selectivity towards dimethyl ether versus methanol.
An even further object of the present invention is to provide a catalyst and process for converting carbon dioxide to oxygenates wherein the amount of carbon monoxide produced is minimized.
A yet further object of the present invention is to provide a novel process for preparing a catalyst effective to convert carbon dioxide into oxygenates.
It should be noted that not all of the above-listed objects need be accomplished by the invention claimed herein and other objects and advantages of this invention will be apparent from the following description of the invention and appended claims.
In accordance with one embodiment of the present invention, a catalyst composition is provided. The catalyst composition comprises copper, zinc, aluminum, gallium, and a solid acid.
In accordance with another embodiment of the present invention a catalyst composition for converting carbon dioxide to methanol and dimethyl ether is provided. The catalyst composition comprises reduced-valence copper, zinc oxide, aluminum oxide, gallium oxide, and a zeolite having an acid function.
In accordance with a further embodiment of the present invention, a method of making a catalyst composition is provided. The method comprises the steps of: (a) combining a copper-containing compound, a zinc-containing compound, an aluminum-containing compound, a gallium-containing compound, and a solid acid compound to form a catalyst mixture; (b) calcining the catalyst mixture to form a calcined catalyst; and (c) reducing the calcined catalyst to form a reduced catalyst.
In accordance with a still further embodiment of the present invention, a process for converting a carbon dioxide-containing feed into oxygenates is provided. The process comprises the steps of: (a) contacting the carbon dioxide-containing feed with a catalyst composition comprising copper, zinc, aluminum, gallium, and a solid acid in a reaction zone under reaction conditions sufficient to convert at least a portion of the carbon dioxide-containing feed into oxygenates; and (b) recovering of at least a portion of the oxygenates from the reaction zone.
In accordance with the first embodiment of the present invention a catalyst comprising copper, zinc, aluminum, gallium, and a solid acid is provided.
The copper component of the inventive composition can exist in the form of any copper-containing compound such as, for example, elemental copper, copper oxide, and/or a copper oxide precursor. It is preferred for the inventive catalyst composition to contain from about 2 to about 50 weight percent copper based on the total weight of the catalyst composition, more preferably from about 10 to about 35 weight percent copper, most preferably from 20 to 30 weight percent copper. At least a substantial portion of the copper component is preferably present in the inventive catalyst composition in a reduced-valence state. The valence of the copper in the reduced-valence state (i.e., the reduced-valence copper) is less than the valence of copper in its common oxidized state, preferably less than two, most preferably zero. It is preferred that at least about 20 weight percent of the copper present in the inventive catalyst composition be present in the reduced-valence state, more preferably at least about 60 weight percent of the copper is present in the reduced-valence state, and most preferably at least 80 weight percent of the copper is present in the reduced-valence state.
The zinc component can exist in the form of any zinc-containing compound such as, for example, elemental zinc, zinc oxide, and/or a zinc oxide precursor. Preferably, the zinc component comprises zinc oxide. It is preferred for the inventive composition to contain from about 1 to about 40 weight percent zinc based on the total weight of the catalyst composition, more preferably from about 5 to about 30 weight percent zinc, most preferably from 10 to 20 weight percent zinc. The weight ratio of zinc to copper in the inventive catalyst composition is preferably from about 0.01:1 to about 10:1, more preferably from about 0.1:1 to about 5:1, and most preferably from 0.4:1 to 0.8:1.
The aluminum component can exist in the form of any aluminum-containing compound such as, for example, elemental aluminum, aluminum oxide, and/or an aluminum oxide precursor. Preferably, the aluminum component comprises aluminum oxide. It is preferred for the inventive catalyst composition to contain from about 0.5 to about 25 weight percent aluminum based on the total weight of the catalyst composition, more preferably from about 1 to about 15 weight percent aluminum, and most preferably from 2 to 5 weight percent aluminum. The weight ratio of aluminum to copper in the inventive catalyst composition is preferably from about 0.01:1 to about 5:1, more preferably from about 0.05:1 to about 1:1, and most preferably from 0.1:1 to 0.2:1.
The gallium component can exist in the form of any gallium-containing compound such as, for example, elemental gallium, gallium oxide, and/or a gallium oxide precursor. Preferably, the gallium component comprises gallium oxide. It is preferred for the inventive catalyst composition to contain from about 0.1 to about 15 weight percent gallium based on the total weight of the inventive catalyst composition, more preferably from about 0.5 to about 10 weight percent gallium, and most preferably from 1 to 3 weight percent gallium. The weight ratio of gallium to copper in the inventive catalyst composition is preferably from about 0.005:1 to about 5:1, more preferably from about 0.01:1 to about 1:1, and most preferably from 0.05:1 to 0.2:1.
The xe2x80x9csolid acidxe2x80x9d of the inventive composition is defined herein as being any solid compound which exhibits an acid function (i.e., functions as a proton donor). Preferably, the solid acid is a zeolite. More preferably, the solid acid is a zeolite having a constraint index (as defined in U.S. Pat. No. 4,097,367, which is incorporated herein by reference) in the range of from about 0.4 to about 12, preferably from 2 to 9. Generally, the molar ratio of SiO2 to Al2O3 in the crystalline framework of the preferred solid acid zeolite is about 5:1 and can range up to infinity. Preferably, the molar ratio of SiO2 to Al2O3 in the preferred solid acid zeolite framework is from about 5:1 to about 200:1, more preferably from 40:1 to 70:1. Preferred solid acid zeolites include ZSM-5, ZSM-8, ZSM-11, ZSM-12, ZSM-35, ZSM-38, and mixtures thereof. The preferred solid acid zeolites can also be described as crystalline aluminosilicates. Suitable aluminosilicates generally have a medium pore size, generally being an effective fine porous size of from about 5 to about 6.5 angstroms, although small (generally 3 to 5 angstroms) or large pore (generally 7 to 8 angstroms) solid acid zeolites can also be used. ZSM-5 and similar solid acid zeolites that have been identified as having a framework topology identified as MFI are particularly preferred because of their shape selectivity. The presently most preferred solid acid is ZSM-5.
It is preferred for the inventive catalyst composition to contain from about 5 to about 80 weight percent solid acid based on the total weight of the inventive catalyst composition, more preferably from about 10 to about 60 weight percent solid acid, and most preferably from 30 to 50 weight percent solid acid. The weight ratio of solid acid to copper in the inventive catalyst composition is preferably from about 0.1:1 to about 20:1, more preferably from about 0.5:1 to about 10:1, and most preferably from 1:1 to 2:1.
The inventive catalyst composition can further comprise a binder component having binding properties which provide for a final catalyst composition having the desired physical properties. Examples of suitable binder materials include those selected from the group consisting of bentonite, aluminate, kaolin, alumina, silica, colloidal silica, sodium silicate, titania, zirconia, aluminosilicates (e.g., clays), zinc aluminate, zinc titanate, metal oxides, and any mixtures thereof. Preferred binders are selected from the group consisting of bentonite, alumina, silica, aluminosilicates, and any two or more thereof. When the inventive catalyst composition comprises a binder, it is preferred for the catalyst composition to contain from about 1 to about 40 weight percent binder based on the total weight of the inventive catalyst composition, more preferably from about 4 to about 30 weight percent binder, and most preferably from 8 to 15 weight percent binder. The weight ratio of binder to copper in the inventive catalyst composition is preferably from about 0.02:1 to about 10:1, more preferably from about 0.1:1 to about 5:1, and most preferably from 0.2:1 to 1:1.
It is preferred for the copper, zinc, aluminum, and gallium components of the inventive catalyst composition to be present in the form of a coprecipitate with the coprecipitate and the zeolite, and optionally the binder, being substantially evenly dispersed throughout the final catalyst composition.
In accordance with the second embodiment of the present invention a method of making the inventive catalyst composition of the first embodiment of the present invention is provided.
The inventive catalyst composition is preferably made, at least in part, by a coprecipitation process. The coprecipitation process can be commenced by contacting an aqueous metal-salt solution containing a copper compound, a zinc compound, an aluminum compound, a gallium compound, and water with a basic substance under suitable conditions to cause coprecipitation of the metals. After the solid coprecipitate has formed, it is allowed to age for a suitable period of time. The coprecipitate is then filtered from the remaining liquid solution, washed, dried, and calcined.
The aqueous metal-salt solution employed in the above-described coprecipitation process is preferably formed by combining soluble salts of copper, zinc, aluminum, and gallium with an appropriate amount of a suitable solvent. The soluble salts of copper, zinc, aluminum, and gallium can be, for example, nitrates, acetates, halides, or any other suitable salts known to those skilled in the art to form the desired product. Most preferably, the metal-salt solution is formed by combining copper nitrate, zinc nitrate, aluminum nitrate, gallium nitrate, and water in appropriate amounts. Preferably, the ratio of zinc to copper in the aqueous metal-salt solution is from about 0.01:1 to about 10:1, more preferably from about 0.1:1 to about 5:1, and most preferably from 0.4:1 to 1:1. Preferably, the weight ratio of aluminum to copper in the aqueous metal-salt solution is from about 0.01:1 to about 5:1, more preferably from about 0.05:1 to about 1:1, and most preferably 0.1:1 to 0.2:1. Preferably, the weight ratio of gallium to copper in the aqueous metal-salt solution is from about 0.005:1 to about 5:1, more preferably from about 0.01:1 to about 1:1, and most preferably from 0.05:1 to 0.2:1. The amount of solvent, preferably water, employed in the solution can be any amount of solvent sufficient to dissolve the metal salts. Preferably, the weight ratio of solvent to the copper in the aqueous metal-salt solution is from about 0.1:1 to about 1,000:1, more preferably from about 1:1 to about 100:1, and most preferably from 5:1 to 20:1.
The basic substance contacted with the aqueous metal-salt solution can be any substance operable to facilitate coprecipitation of a Cu/Zn/Al/Ga solid coprecipitate. The basic substance is preferably a liquid substance having a pH value of more than about 8, preferably between about 9 and 12. The basic substance preferably comprises a basic component selected from the group consisting of ammonia, carbonates, bicarbonates, and alkali metal hydroxides. Most preferably, the basic substance is an aqueous solution containing sodium bicarbonate and water, with the weight ratio of sodium bicarbonate to water in the solution being from about 0.01:1 to 10:1, most preferably from 0.05:1 to 0.2:1.
The aqueous metal-salt solution is preferably contacted with the basic solution by combining the basic substance and the metal-salt solution in a mixer. Preferably, after combining the basic substance and the metal-salt solution, the resulting solution is mixed by any suitable method known in the art for a period of about 5 minutes to about 5 hours, more preferably from about 10 minutes to about 1 hour. During mixing of the metal-salt solution and the basic substance, it is preferred that the mixed solution be maintained at a temperature of from about 10xc2x0 C. to about 200xc2x0 C., more preferably from about 20xc2x0 C. to about 100xc2x0 C., and most preferably from about 60xc2x0 C. to about 80xc2x0 C.
After mixing, the formed coprecipitate and liquid solution are preferably aged, without mixing, at a temperature of from about 20xc2x0 C. to about 100xc2x0 C. for a time period of from about 10 minutes to about 5 hours.
The solid coprecipitate formed during the mixing and aging of the solution is preferably separated from the liquid solution by any means known in the art such as, for example, filtering. The separated solid coprecipitate is then preferably washed to remove trace amounts of the liquid solution therefrom. After washing, the solid coprecipitate is preferably dried at a temperature of from about 80xc2x0 C. to about 200xc2x0 C., more preferably from 100xc2x0 C. to 140xc2x0 C. for a period of from about 0.5 to about 10 hours, more preferably from about 1 to about 5 hours. Thereafter, the dried solid coprecipitate is preferably calcined at a temperature of from about 200xc2x0 C. to about 600xc2x0 C., more preferably from about 300xc2x0 C. to about 400xc2x0 C. for a period of from about 1 to about 20 hours, more preferably from about 2 to about 8 hours.
The calcined coprecipitate comprising copper, zinc, aluminum, and gallium is thereafter preferably physically mixed with the solid acid and the binder in appropriate proportions suitable for providing the inventive catalyst composition described in the first embodiment of the present invention. The solid coprecipitate is preferably crushed into fine particles prior to or during mixing with the solid acid and binder. Water can be added during the physical mixing of the solid coprecipitate, solid acid catalyst, and binder to thereby form a paste capable of being shaped into suitable catalyst particles. Preferably, the paste is thereafter extruded into extrudate having a diameter of from about {fraction (1/16)} to about xe2x85x9 of an inch. Other suitable methods of shaping such as, for example, granulizing, pelletizing, and sphering may also be employed.
After shaping the mixture of solid coprecipitate, solid acid, and binder, the shaped catalyst is preferably dried at a temperature of from about 80xc2x0 C. to about 200xc2x0 C. for a period of from about 1 to about 10 hours. The dried catalyst is then calcined at a temperature of from about 200xc2x0 C. to about 600xc2x0 C., preferably from 300xc2x0 C. to 400xc2x0 C., for a period of from about 1 to about 10 hours, preferably from 3 to 7 hours.
After calcining, the catalyst is preferably reduced under conditions sufficient to reduce the valence of the copper component of the catalyst. Preferably, reduction is accomplished by contacting the catalyst with hydrogen at a temperature of from about 180xc2x0 C. to about 320xc2x0 C., preferably from 200xc2x0 C. to 280xc2x0 C., for a period of from about 0.5 to about 20 hours, preferably from 2 to 8 hours.
In accordance with a third embodiment of the present invention a process for converting carbon dioxide to oxygenates using the inventive catalyst composition described in the first embodiment of the present invention is provided.
The catalyst described and prepared in accordance with the first and second embodiments of the present invention can be employed to convert a feed containing carbon dioxide into oxygenates under any suitable conditions effective to promote such conversion. The carbon dioxide-containing feed and catalyst can be contacted in the reaction zone of any suitable reactor known in the art such as, for example, a fixed bed reactor, a moving bed reactor, a fluidized bed reactor, and transport reactors. Preferably, the inventive catalyst and the carbon dioxide-containing feed are contacted in a fixed bed reactor.
The carbon dioxide-containing feed contacted with the inventive catalyst composition preferably comprises hydrogen and carbon dioxide in amounts such that the hydrogen to carbon dioxide volume ratio (H2:CO2) of the feed is in the range of from about 0.1:1 to about 100:1, more preferably from about 1:1 to about 50:1, and most preferably from 5:1 to 15:1. Although the carbon dioxide-containing feed can contain trace amounts of other compounds such as, for example, carbon monoxide, it is preferred that the carbon dioxide-containing feed consist essentially of hydrogen and carbon dioxide. Preferably, the volume percent of carbon dioxide in the carbon dioxide-containing feed is from about 1 to about 50 volume percent by volume of the total carbon dioxide-containing feed, more preferably from about 5 to about 20 volume percent, and most preferably from 8 to 15 volume percent. The volume of hydrogen contained in the carbon dioxide-containing feed is preferably from about 50 to about 99 volume percent hydrogen based on the total volume of the carbon dioxide-containing feed, more preferably from about 70 to about 95 volume percent hydrogen, and most preferably from 85 to 95 volume percent hydrogen.
The carbon dioxide-containing feed preferably comprises less than about 20 volume percent of compounds other than carbon dioxide and hydrogen based on the total volume of the carbon dioxide-containing feed, more preferably less than about 10 volume percent of compounds other than carbon dioxide and hydrogen, even more preferably less than 2 volume percent of compounds other than carbon dioxide and hydrogen.
The rate at which the carbon dioxide-containing feed is charged to the reactor can be any rate suitable for promoting the conversion of carbon dioxide to oxygenates, particularly dimethyl ether. Preferably, the gas hourly space velocity (GHSV) of the carbon dioxide-containing feed charged to the reactor is in the range of from about 100 to about 20,000 hrxe2x88x921, more preferably from 1,000 to 5,000 hrxe2x88x921.
The temperature and pressure at which the reaction zone of the reactor is maintained can be any temperature and pressure which optimizes the conversion of carbon dioxide to oxygenates, particularly dimethyl ether. The reaction zone is preferably maintained at a temperature of from about 150xc2x0 C. to about 500xc2x0 C., more preferably from about 200xc2x0 C. to about 400xc2x0 C., and most preferably from 250xc2x0 C. to 320xc2x0 C. The pressure in the reaction zone is preferably from about 200 psig to about 5,000 psig, more preferably from about 500 psig to about 4,000 psig, and most preferably from 1,200 psig to 1,600 psig.
After contacting the carbon dioxide-containing feed and the catalyst composition under reaction conditions, the oxygenate products can be recovered from the reactor by any suitable means known in the art.
The product of the reaction preferably comprises dimethyl ether (DME), methanol, and carbon dioxide. The amount of DME in the product as a mole percentage of all the carbon in the product is preferably greater than about 5 carbon mole percent, more preferably greater than about 20 carbon mole percent, and most preferably greater than 60 carbon mole percent. The amount of methanol in the product as a mole percentage of all the carbon in the product is preferably less than about 50 carbon mole percent, most preferably less than 25 carbon mole percent. The amount of carbon monoxide in the product as a mole percentage of all the carbon in the product is preferably less than about 20 carbon mole percent, more preferably less than about 10 carbon mole percent, and most preferably less than 5 carbon mole percent.